Single receptacle to a double receptacle

I have a single receptacle in my garage that is used for my sprinkler system. I would like to change it to a double to plug in my garage fridge as well. Is this ok to do? When I did it I noticed only one of each wire (black,white, green) coming out of the wall. Should I use a GFCI? Will this cause any problems?? Thanks

Depending on how old the house is, that recepticle might already be controlled (protected) by a GFCI somewhere else. If so, it is supposed to have a sticker on it, but people often neglect to do that or they come off.

If it isn't already protected, then you should add one, then use the load connections on it to power the second recepticle...no need to buy two as long as the one you do buy has load connections. The incoming power goes to the line connections.

receptacle circuits in garages dont need gfi if they are not readly accessible or single receptacles that in your case only serve a icebox thats not easily moved to plug something else in according to the 2005 nec code book now if you add a double receptacle and make it so its easy to plug a extension cord in or anything else then it MUST be GFI protected

If the fridge doesn't have a grounded plug you should add one in case there's current leakage to the case of the fridge. You'd need to run the ground wire to the metal fridge housing, under a screw. Be sure to scrape the paint so you get a good electrical connection.

Otherwise the fridge could be or become a shock hazard.

If you plug in the new grounded plug and it trips a breaker the fridge is already a shock hazard.